1. Field of the Invention
The present invention relates to an optical coherent tomography system, using a tunable surface emission laser source in a single mode.
2. Discussion of the Related Art
In association with advancement of a medical technology such as an endoscopic therapy, a diagnostic measure for noninvasively diagnosing a pathologic tissue in real time has been desired recently. For example, an electronic endoscope using a CCD, and an imaging technology using a CT, an MRI, and an ultrasonic wave are used in a diagnostic approach. The electronic endoscope is only used for observing a biological surface, and the latter diagnostic imaging system has a technical limitation to observe with the micron order resolution. As a technology for complimenting the measure, an optical coherence tomography system (OCT) has drawn attention.
There are two types of the OCT, namely a time domain OCT (TD-OCT) and a frequency domain OCT-(FD-OCT), and there are two types of the FD-OCT, namely a spectrometer type (SD-OCT) and a swept source type (SS-OCT). The time domain OCT sheds broadband light on a living body and analyzes frequencies of interference components in the reflecting light from the living body. However, a signal light from a specific depth cannot be detected with high-sensitivity because reflecting lights from different depths in the interference light overlap each other in this method.
Handbook of Optical Coherence Tomography, p41-43, Mercer Dekker, Inc. 2002 shows a tunable type OCT. The system obtains a tomography image through steps of emitting light to a living body, continuously changes wavelength of the emitted light, makes a reference light and reflecting lights returning from different depths in vivo interfere in an interferometer, and analyzes frequency components of the interfering lights. This technology is expected as an advanced system because a tomography image of extremely high-resolution can be constructed through frequency analysis of signals from inside a body. The SS-OCT using a tunable light source is suitable for practical use in such as endoscope because the SS-OCT has high measurement sensitivity and is strong to dynamic noise. Since the broader a range of wavelength scanning of emitted light is, the higher a range of frequency analysis is, a resolution in depth direction is improved.
As a tunable light source using the SS-OCT, there are a multi-mode tunable laser of the fiber-ring type and an end-face wavelength laser of the integration type. However, the fiber-ring type laser has a limitation of measurement depth, since its spectral width becomes wider with accelerating wavelength scanning speed. As a result of this, a coherence length becomes short. In addition, the fiber-ring type cannot be downsized, and has a problem on mass production. In the end-face wavelength laser of the integration type, a wave length can be varied in the single-mode, while high speed scanning is difficult since a phase control for continuous scanning is complicated.